Japanese Coast Research Articles

Length–weight relations of underutilized nine fish species (Actinopterygii) from set-net fishery, Mie, Japan

Bycatch, the unintentional capture of non-target species, poses a significant challenge in fisheries management worldwide. Underutilized bycatch species often lack biological information, hindering the assessment of their stock status and the implementation of appropriate management strategies. This study investigated the length–weight relations (LWRs) of 9 of 14 underutilized bycatch fish species collected from set-net fisheries in Nishiki, Mie Prefecture, Japan, during the 2018–2020 fishing seasons. The following fish species were studied: Hyperoglyphe japonica (Döderlein, 1884); Labracoglossa argenteiventris Peters, 1866; Macroramphosus sagifue Jordan et Starks, 1902; Microcanthus strigatus (Cuvier, 1831); Ostorhinchus semilineatus (Temminck et Schlegel, 1843); Parapriacanthus ransonneti Steindachner, 1870; Rexea prometheoides (Bleeker, 1856); Siganus fuscescens (Houttuyn, 1782); Upeneus japonicus (Houttuyn, 1782). A total of 251 fishing operations were conducted, and the collected fish specimens were measured for length (fork length) and wet weight. Simple regression analysis was performed on the logarithm of length and weight data to estimate the LWR parameters (a, b, and R2) for each species. The estimated values of the relative growth coefficient (b) ranged from 2.862 to 3.311, and the coefficients of determination (R2) were above 0.8 for all species, indicating robust LWR estimations. Notably, five species were not previously listed or credibly described in FishBase, and four were new records for the Japanese coast. This study provides new biological information on LWRs for underutilized bycatch fish species, contributing to our understanding of their population dynamics and supporting future management efforts for sustainable fisheries and biodiversity conservation.

Fisheries shocks provide an opportunity to reveal multiple recruitment sources of sardine in the Sea of Japan

The abrupt decline in sardine catches in the Sea of Japan and the East China Sea (SJ-ECS) in 2014 and 2019 and the recovery in the following years call into question the current assumption that sardines in the SJ-ECS form a self-recruiting subpopulation. To test this hypothesis, we analysed otolith stable oxygen and carbon isotope profiles (δ18O, δ13C) of age-0 and age-1 sardines from 2010 and 2013–2015 year-classes captured in the SJ-ECS, as geographic markers for nursery areas. Age-0 sardines generally showed a significant ontogenetic decrease in otolith δ18O from larval to juvenile stages. However, the majority of age-1 captured in spring 2011, 2015 and 2016 showed non-decreasing otolith δ18O profiles, suggesting that the age-0 off the Japanese coast were not the main source of recruitment. Different migration groups were thus indicated: the “locals” growing up off the Japanese coast and the migrating “nonlocals”. The isotope profiles of the “nonlocals” overlapped with those of age-0 captured in the subarctic North Pacific, suggesting that they may be migrants from the Pacific, or perhaps an unobserved northward migration group in the SJ-ECS. Our results highlight the considerable uncertainty in the population structure assumed in current stock assessment models for Japanese sardine.

Tracing Sediment‐Affected Water Masses From Coastal Areas to Offshore Oceans Using In Situ Sensing of Fluorescent Organic Matter

AbstractThe lateral transport of nutrient‐rich water masses from coastal areas to offshore areas is an important factor controlling the primary production of phytoplankton in offshore regions. If the coastal low‐salinity waters have high nutrient levels, lateral transport can be traced via salinity observations. However, if high level of nutrients in the waters are not coupled with low salinity, for example, input from coastal sediments, water transport cannot be traced via salinity observations. In this study, visible fluorescent organic matter (FOMvis) was monitored by an in situ sensor as a tracer for sediment‐affected water masses from coastal areas. The FOMvis distribution in the upper 500 m water layer in offshore Japan along the 143°E transect was generally controlled by the distributions of subtropical water (i.e., Kuroshio water and Tsugaru warm current water) and subarctic water (i.e., Oyashio water). However, anomalously high levels of FOMvis were observed at depths of 20–60 m at an observational station, which could not be explained by physical mixing. This likely originated from the lateral transport of water masses from coastal areas. In and around Sendai Bay, where observations were carried out as an example of the Japanese coast, high levels of FOMvis were observed, accompanied by negative N*, an indicator of denitrification, and phosphate and silicic acid inputs from anoxic sediments. These observational results imply that FOMvis observation via in situ sensors can be useful for tracing the input of nutrient‐rich sediment‐affected water masses from coastal areas to offshore oceans.

Freshwater spreading far offshore the Japanese coast

River discharge to the ocean influences the transport of salts and nutrients and is a source of variability in water mass distribution and the elemental cycle. Recently, using an underwater glider, we detected thick, low-salinity water offshore for the first time, probably derived from coastal waters, in the central-eastern Sea of Japan, whose primary productivity is comparable to that of the western North Pacific. Thereafter, we aimed to investigate the offshore advection and diffusion of coastal water and its variability and assess their impact. We examined the effects of river water discharge on the flow field and biological production. Numerical experiments demonstrated that low-salinity water observed by the glider in spring was discharged from the Japanese coast to offshore regions. The water is discharged offshore because of its interaction with mesoscale eddies. A relationship between the modeled low-salinity water transport to the offshore region and the observed chlorophyll-a in the offshore region was also observed, indicating the influence of river water on offshore biological production. This study contributes to understanding coastal-offshore water exchange, ocean circulation, elemental cycles, and biological production, which are frontiers in the Sea of Japan and throughout the world.

Contrasting population genetic structure of three semi‐terrestrial brachyuran crabs on the coast of the Japanese archipelago

AbstractAnthropogenic activities have reduced ecotones between the ocean and land, which is likely to threaten the population of brackish‐water brachyuran crabs. To assess the current status of these crabs, we examine the population genetic structures of three semi‐terrestrial brachyuran crabs widely distributed along the coast of the Japan and to clarify factors determining their genetic structures. We collected 184 Orisarma dehaani, 252 Chiromantes haematocheir, and 151 Helice tridens crabs from 36 localities of the Japanese archipelago. Genome‐wide SNP data from these crabs were analyzed using MIG‐seq. Bayesian clustering of STRUCTURE and DAPC analysis were used to identify genetically disturbed populations and to visualize genetic differentiation between local populations. Genetic population structure showed clear differentiation between populations on the Pacific coast of the Tohoku region and on other Japanese coasts in O. dehaani, but not in C. haematocheir or H. tridens. The inbreeding coefficient of O. dehaani was significantly higher on the Pacific coast of the Tohoku region compared to other Japanese coasts. C. haematocheir and H. tridens had homogeneous genetic structures along the Japanese coast, but showed genetic differentiation of a local population at their range limits. Thus, O. dehaani showed little gene flow and clear genetic differentiation between populations in the Tohoku Pacific region and those on other Japanese coasts due to ocean currents. Although such a regional differentiation was not found in C. haematocheir and H. tridens, one population of C. haematocheir was genetically isolated at the edge of its distribution range and likely vulnerable to environmental changes.

Microbial Detoxification of Sediments Underpins Persistence of Zostera marina Meadows.

Eelgrass meadows have attracted much attention not only for their ability to maintain marine ecosystems as feeding grounds for marine organisms but also for their potential to store atmospheric and dissolved CO2 as blue carbon. This study comprehensively evaluated the bacterial and chemical data obtained from eelgrass sediments of different scales along the Japanese coast to investigate the effect on the acclimatization of eelgrass. Regardless of the eelgrass habitat, approximately 1% Anaerolineales, Babeliales, Cytophagales, and Phycisphaerales was present in the bottom sediment. Sulfate-reducing bacteria (SRB) were present at 3.69% in eelgrass sediment compared to 1.70% in bare sediment. Sulfur-oxidizing bacteria (SOB) were present at 2.81% and 1.10% in the eelgrass and bare sediment, respectively. Bacterial composition analysis and linear discriminant analysis revealed that SOB detoxified H2S in the eelgrass meadows and that the larger-scale eelgrass meadows had a higher diversity of SOB. Our result indicated that there were regional differences in the system that detoxifies H2S in eelgrass meadows, either microbial oxidation mediated by SOB or O2 permeation via the physical diffusion of benthos. However, since bacterial flora and phylogenetic analyses cannot show bias and/or causality due to PCR, future kinetic studies on microbial metabolism are expected.

Nation-scale multidecadal shoreline extraction and coastal spatio-temporal change monitoring using cross-mission remote sensing data

Mapping shorelines is critical for coastal management and conservation, as shorelines are essential indicators for understanding coastal geomorphology and dynamics. Nonetheless, as shorelines are often lengthy and rugged, in-situ surveys are labor-intensive and costly. Oppositely, spaceborne remote sensing data provide cost-efficient alternatives, yet, most earlier studies rely on manual digitization or thresholding band ratio-based water indices to delineate instantaneous shorelines, which locations are affected by varying water color and tides. Consequently, cross-temporal assessments of environmental and anthropogenic impacts on coastal stability remain challenging. The present study develops an edge detection-based, dual-temporal, nationwide, tide-coordinated, and sediment cell-based shoreline dataset in Japan, i.e., the TSSD-JP. Both 1980s and 2020s shorelines along the Japanese coasts are extracted using cross-mission multispectral data and an automated shoreline extraction method. The results are validated with national coast surveys and global shoreline databases, confirming a positional accuracy better than 80% at 50 m. Moreover, founded on the extracted dual-temporal shorelines, 30+ year coastal changes are measured. An increased land area of 293.21 km2 at a rate of 8.50 km2/yr is estimated. Evident accretions in bay regions of southern Honshu due to extensive land reclamation and engineering constructions are observed. Since the analysis is conducted on natural management units with a physical basis, potential biases caused by jurisdictional boundaries are overcome. Practically, because TSSD-JP is derived from freely accessible data and tools, its universal applicability is guaranteed. Thence, the proposed approach can be applied to different nations' intertidal zones to assess the coastal vulnerability and resilience as well as support local governments’ shoreline management plans (SMPs).

Distribution of tetrodotoxin and its analogs in the toxic flatworm Planocera multitentaculata from Honshu Island, Japan

Tetrodotoxin (TTX), one of the most potent naturally occurring neurotoxins, has been detected in both marine and terrestrial organisms. The marine polyclad flatworm Planocera multitentaculata is known to contain high levels of TTX throughout its life cycle, including in the egg and larval stages. We recently reported that P. multitentaculata from the coastal area of Hayama, Kanagawa, Japan, possesses TTX analogs including 5,6,11-trideoxyTTX, monodeoxyTTXs, dideoxyTTXs, and 11-norTTX-6(S)-ol. It is not known whether all P. multitentaculata occurring in the Japanese archipelago possess major TTX analogs, or whether regional variations in TTX profile exist. In this study, we used high-resolution liquid chromatography–mass spectrometry to investigate the content profile of TTX and its analogs in P. multitentaculata collected from several regions of the Japanese coast. We detected TTX and its major analogs 5,6,11-trideoxyTTX, dideoxyTTXs, monodeoxyTTXs, and 11-norTTX-6(S)-ol in all specimens regardless of the sampling site. In addition, their relative proportions showed a similar pattern. These results suggest that the flatworm may have mechanisms involved in maintaining the proportions of TTX and its analogs in the body. How the toxic flatworms acquire the highly concentrated toxins and maintain the proportions of TTXs is therefore of great interest and further investigation.

Contribution of environmental factors to genetic variation in the Pacific white-sided dolphin

Environmental changes driven by pollution and climate change have affected many species globally. The Pacific white-sided dolphin, or Lagenorhynchus obliquidens (L. obliquidens), is threatened by water pollution, which is the product of human actions and can impact the genetic structure of species in affected areas. While researchers have studied genetic differences between populations of L. obliquidens in Japan, they failed to explore the potential environmental drivers of these differences. Our research aimed to identify the major environmental factors impacting the genetic variation of L. obliquidens, which will increase our understanding of how this species responds to changes in the environment. We studied several different variables, including salinity, ocean currents, temperature, and microplastic levels. Due to the significant presence and variation of microplastics in the ocean surrounding Japan, we hypothesized that this would have the most notable impact on the genetic variation of L. obliquidens. We began our research by gathering genetic sequencing data from a preexisting study focusing on populations along the Japanese coast. We collected environmental data and analyzed it alongside a genetic distance matrix, leveraging data analysis and machine learning tools in R. We found that the most impactful variable was current stability, which did not match our original prediction of microplastic levels having the most significant impact. We then investigated why ocean currents may have had such a strong impact on L. obliquidens and proposed that it has a connection to nutrient and prey distribution, which can be majorly impacted by changes in ocean currents.

Extreme Sea Level Variations in the Sea of Japan Caused by the Passage of Typhoons Maysak and Haishen in September 2020

This study investigates extreme sea level variations recorded by tide gauges in the Sea of Japan during the passages of typhoons Maysak and Haishen in September 2020. Specific focus is on storm surges, seiches and infragravity waves identified in the records using tidal and statistical analyses. At most locations, storm surges, formed through the combined effect of atmospheric pressure changes and strong winds, were the major contributors to the extreme sea level events. For the Russian and Japanese coasts, the first typhoon, Maysak, turned out to have the greatest impact, while for the southeastern coast of the Korean Peninsula, the largest sea level variations were caused by the second typhoon, Haishen. At all sites, the passing storms induced eigen oscillations in the respective bays and harbours. Pronounced high-frequency oscillations with periods of two to seven minutes, which appear to be associated with infragravity waves, were recorded, reaching a maximum range of 2 m at Preobrazheniye. The observed differences in component content and associated characteristics of the observed sea level variations arise from differences in the topographic features of the corresponding coastal sites. Statistical analysis of the atmospheric pressure and wind speed series taken from weather stations and from the ERA5 reanalysis shows that atmospheric pressure is reproduced by reanalysis with high accuracy, whereas wind speed at different stations has significant discrepancies related to differences in orographic features.

TSUNAMI MODELLING IN A BUILT-IN COASTAL ENVIRONMENT WITH ADAPTIVE MESH RIFINEMENT AND VARYING BOTTOM FRICTION

The great Japan earthquake on March/11/2011provoked an extreme tsunami wave toward the northeastern Japanese coast. The tsunami inundation covered a wide range of territories stretching over 500 km2 and caused devastating influences that claimed more than 15,000 lives (Mori et al. (2011)). Miyagi Prefecture, the closest mainland to the epicentre, received massive tsunami waves that reached over 40 m in run-up height. Prasetyo et al. (2019) aimed to reproduce the tsunami wave inundation in Onagawa Town of Miyagi Prefecture by building a physical model using Hybrid Tsunami Open Flume in Ujigawa Open Laboratory (HyTOFU) at Kyoto University to study the inundation process by measuring the wave height and arrival time. For that purpose, two wave types were used, the (Pump-type) hydraulic bore, and the (Piston-type) solitary wave. Also, 2D and quasi-3D (Q3D) models were tested for their robustness and applied to emulate tsunami propagation in the complex coastal city model. In this study, we adopted adaptive mesh refinement (AMR) for its efficiency in tsunami modelling with Basilisk open-source flow solver for non-linear shallow water (Saint-Venant) equations and fully non-linear Boussinesq (Green-Naghdi) equations (Popinet (2015)). The numerical experiment was conducted to validate the model by comparing the wave height with the physical model experiment, the 2D, and Q3D model results by Prasetyo et al. (2019).

Characteristics of mud flocs in the subseabed methane hydrate reservoir and behaviours in flowing water

Recently, it was found that mud flocculates, or flocs, coexist with methane hydrate within turbidite layers off the Japanese coast. There is a concern that mud flocs may be broken down by the flow of pure water generated from the dissociation of methane hydrate, causing sand troubles in the production wells. To clarify the characteristics of the flocs, in this study, the samples of mud flocs collected from natural cores having contained methane hydrate were observed before and after sonication in a pure-water ultrasonic bath. Then, these flocs were placed in distilled water flow to investigate their mechanical stability. As a result, it was confirmed that the flocs mainly consist of silt together with a little clay and fine sand. In addition, these flocs were found to be stable for flow rates on the order of magnitude of 10−3 m/s, which is expected inside the strata during gas production from methane hydrate. However, it was also found that, after the distilled water flow experiments, the flocs were decomposed into silt and clay particles when pressed with a handheld needle in the air, suggesting that the floc might be a source of the fine particles as underground stress conditions change, although not many flocs are likely to be immersed in pure water alone.

Theama japonica sp. nov., an Interstitial Polyclad Flatworm Showing a Wide Distribution along Japanese Coasts.

We establish a new interstitial polyclad species, Theama japonica sp. nov., based on specimens collected from coarse-sandy habitats in three Japanese main islands (Hokkaido, Honshu, and Shikoku) along the coasts of the Pacific Ocean and the Sea of Japan. Theama japonica is characterized by i) two pairs of cerebral eyespots and four to six precerebral eyespots; ii) eosinophilic secretion glands distributed in the distal half of the inner ventral part of the prostatic vesicle; iii) a conical penis papilla, bent up dorsally, with a sclerotized inner wall; iv) the prostatic sheath with an inner angular fold on the dorso-distal side; and v) the external cilia longer dorsally than ventrally. Partial sequences of the cytochrome c oxidase subunit I (COI) gene from 20 specimens collected at eight localities along Japanese coasts represented 19 haplotypes. The uncorrected p-distances among these COI haplotypes fell within intraspecific variations observed in other polyclads. A network analysis based on these COI haplotypes suggested a geographically non-cohesive genetic structure of the species, possibly indicating the species' high dispersibility. Molecular phylogenetic analyses based on a concatenated dataset of 18S and 28S rDNA sequences showed T. japonica formed a clade with other Theama species. The resulting tree also indicates that our new species is more closely related to Theama sp. from Colombia than species from Panama and Croatia.

Morphology and molecular phylogeny of the marine diatom genus Nagumoea (Bacillariophyceae) from Japan

SUMMARYThe canal‐bearing diatom genus Nagumoea, described based on only morphological evidence, was tentatively assigned to the order Bacillariales, although its phylogenetic position remained unclear. Because three isolates of Nagumoea (SK002, SK024 and SK053) were successfully established from Japanese coasts, we performed their morphological observations and molecular phylogenetic analyses to discuss the phylogeny and taxonomic position of this genus. Strains SK002 and SK024 were identified as Nagumoea africana, whereas SK053 conformed with Nagumoea serrata. There was high interspecific divergence between N. africana and N. serrata in the rbcL sequences (8.03–8.17%), indicating their distinctness. Furthermore, intraspecific variations were detected within N. africana (2.35%) in the rbcL, implying its cryptic diversity. The maximum likelihood and Bayesian phylogenetic trees inferred from the plastid rbcL, psbC and nuclear 18S rDNA genes recovered Nagumoea as monophyletic with strong statistical support and embedded within an unresolved, poorly supported lineage containing Achnanthes, Craspedostauros, Staurotropis and Undatella in the canal‐bearing order Bacillariales (= the family Bacillariaceae). Although the constrained tree based on the monophyly of Nagumoea and the other canal‐bearing clade (Surirellales and Rhopalodiales) was statistically rejected by the topology tests, the phylogenetic position of Nagumoea with other Bacillarialean members remains equivocal. The possession of two plastids positioned fore and aft, observed in the present study, and lack of keel, typical of the Bacillariales, indicate the possibility of Nagumoea being part of the ingroup of the Bacillariales or its closely related outgroup.

A state‐space model to estimate spawning rates in a Ruditapes philippinarum population

AbstractThe Ruditapes philippinarum catch in Japan has drastically declined and its recovery is a major challenge. The spawning rates and egg output need to be estimated to discuss stock management strategies. However, it is difficult to observe these directly in a field survey. This study proposes a state‐space model to estimate spawning rates in an R. philippinarum population using the number of individuals at each developmental stage based on traditional histological observations of the gonads. An example of its application to monthly observed data of an R. philippinarum population on the western Japanese coast is also presented. The spawning seasons were quantitatively detected in spring and fall, and the durations were estimated to be 2.8 months in spring and 3.0–3.5 months in fall, based on the spawning rates. The mean condition factor was lower in fall than in spring, but females with a high condition factor were also found in fall. The egg output of these individuals during the fall may be large. The spawning frequency per individual was estimated to be 6.5 times per year based on the model, indicating that R. philippinarum is an r‐strategy species capable of multiple spawning. Questioning how the spawning rates vary with individual size/age and environmental conditions is essential for estimating egg output in R. philippinarum populations. The state‐space model in this study can contribute to the quantification of spawning rates. The results will contribute to estimating the egg output and stock management strategies.

Mechanism of a meteorological tsunami reaching the Japanese coast caused by Lamb and Pekeris waves generated by the 2022 Tonga eruption

Tide gauge data indicated that ocean waves were generated by the eruption of the Hunga Tonga-Hunga Ha’apai volcano on January 15, 2022. These high-amplitude waves reached the Japanese coast earlier than expected for free ocean waves. These fluctuations in sea level were reasonably reproduced by an ocean model, with realistic bottom topography forced by sea-level pressure perturbations well reproduced by atmospheric general circulation models. While early arrival times are associated with atmospheric Lamb waves, the amplitude of the sea level changes cannot be fully explained by the resonance mechanism associated with sea-level pressure fluctuations of Lamb and Pekeris waves alone. Rather, resonance due to atmospheric waves in conjunction with the amplification effects caused by a steep continental slope is required in order to reproduce sea level fluctuations along the Japanese coast. In particular, the deep Northwest Pacific Basin to the east of Japan is important for strengthening the amplitude of the meteorological tsunami.

Effects of Anthropogenic Forcings on Multidecadal Variability of the Sea Level Around the Japanese Coast Simulated by MRI‐ESM2.0 for CMIP6

AbstractThe observed sea level (SL) around the Japanese coast shows a peculiar multidecadal variation with the peak in the 1950s followed by the gradual fall until the 1970s and the rebound continuing to the present, making the recent SL rise less remarkable in the historical record. The multidecadal SL variability of an ensemble mean of the Coupled Model Intercomparison Project phase 6 historical simulations using the Meteorological Research Institute Earth System Model version 2.0 (MRI‐ESM2.0) compares well with the observation and is greater than the piControl one, implying the observed variability could be a forced one. The MRI‐ESM2.0 simulations for the Detection and Attribution Model Intercomparison Project suggest the increase in anthropogenic aerosols and greenhouse gases caused the fall and rise of the SL, respectively. Additional sensitivity runs indicate the surface heat loss in the North Pacific due to anthropogenic aerosols plays a dominant role in the SL fall.

A new species of the ghost shrimp genus Callianassa Leach, 1814 (Decapoda: Axiidea: Callianassidae) from Wakasa Bay, western Japan: the first representative of the genus from the Pacific region.

The ghost shrimp genus Callianassa Leach, 1814 is currently represented by six taxa, of which five are known from the eastern Atlantic and only C. persica Sakai, 2005 from the Persian Gulf. In this study, a new species of Callianassa is described and illustrated on the basis of material from the subtidal soft sediments (at 2628 m depth) in Wakasa Bay, western Japanese coast of the Sea of Japan. Callianassa ogurai n. sp. is the first species of the genus from the Pacific region. This study discusses characters differentiating the new species and other five congeneric species. Molecular phylogenetic analysis using the mitochondrial 16S rRNA gene supports the close relationship between the new species and C. subterranea (Montagu, 1808), warranting the generic assignment of the new species to Callianassa sensu stricto. We also explore the contentious taxonomic status of C. timiris (Sakai, Trkay, Beck Freiwald, 2015).

Sea level variability along the Japanese coast forced by the Kuroshio and its extension

Sea level variability along the Japanese coast and its relation to the Kuroshio-Kuroshio Extension (KE) are investigated using ocean reanalysis data. The first mode of an empirical-orthogonal-function for the coastal sea-level represents a simultaneous sea-level change along the whole Japanese coast, which is synchronized with sea levels in the KE region , the Japan Sea and the East China Sea. The second mode is characterized by an east–west dipole pattern at the south coast. The first mode is correlated with the Kuroshio variations around the Izu–Ogasawara Ridge (IOR) and tends to be in a positive phase when the Kuroshio takes a nearshore path around IOR. The Kuroshio’s position around IOR is closely related to the KE dynamic state. When the KE jet is in a stable (unstable) state, a nearshore (meandering) Kuroshio path is formed around IOR. A composite analysis suggests that the sea level along the Japanese coast becomes high due to propagation of coastal trapped waves when the Kuroshio takes a nearshore path around IOR. That is why the first mode is synchronized with the KE decadal variability. The second mode has a close relation with the Kuroshio Large Meander (LM). The eastern positive anomaly at the coast between the Izu and Kii Peninsulas is formed by warm Kuroshio water brought by a westward branch flow along the coast. The western negative anomaly is attributed to a southward shift in the Kuroshio south of the Kii Peninsula associated with the LM.

Another mite species discovered via social media - Ameronothrus retweet sp. nov. (Acari, Oribatida) from Japanese coasts, exhibiting an interesting sexual dimorphism

ABSTRACT The recent discovery of the oribatid mite Ameronothrus twitter via a social media platform resulted in a considerable media response and raised the awareness of the public for these tiny organisms. As a direct consequence, another new marine associated ameronothroid species was now discovered via the same social media service. Moreover, the record of this new species, Ameronothrus retweet sp. n., represents the first report of an ameronothroid taxon from the Sea of Japan coast, indicating that this coastline was successfully colonized by these organisms and that further species could be present there. Ameronothrus retweet sp. n. shows an extraordinary sexual dimorphism with females having relatively shorter legs and a strongly folded notogastral integument. Based on morphological characteristics, it is suggested that the new species is closest related to Ameronothrus lineatus and Ameronothrus nigrofemoratus. A molecular genetic investigation of selected ameronothroid taxa, using the 18S rRNA gene, shows that all Ameronothrus species are closely related and represent a distinct monophyletic genus. In a larger phylogenetic context, the Fortuyniidae and Selenoribatidae are given as sister taxa with a monophyletic origin, whereas certain members of Ameronothridae are placed in paraphyletic positions, supporting theories of an independent origin of the marine associated lifestyle in ameronothroid mites. http://www.zoobank.org/urn:lsid:zoobank.org:pub:D0D9B80E-D144-436D-82B1-F2847178E861 http://www.zoobank.org/urn:lsid:zoobank.org:act:0CFB4401-1EC4-4BB8-97CD-13812FF11D83